Orbital booster idea

I had an idea several years ago that I think is worth writing up. It is for a system to lift any kind of cargo from a low orbit around a planet into a higher one, with no expenditure of fuel.

Design

The system consists of two carriers: one shaped like a cylinder with a hole bored through it and the other shaped like a cigar. The cigar must be able to pass straight through the hole in the cylinder. The two must have the same mass, after being loaded with whatever cargo is to be carried. This could be achieved by making the cylinder fairly thin, by making the cigar longer than the cylinder, or by having the latter denser than the former. Within the cavity of the cylinder are a series of electromagnets. Likewise, under the skin of the cigar. Around the cylinder is an array of photovoltaic panels. Likewise, on the skin of the cigar. Each contains a system for storing electrical energy.

In addition to these main systems, each unit would require celestial navigation capability: the ability to determine its position in space using the observation of the starfield around it, as modern nuclear warheads do. This would allow it to act independently of ground-based tracking or the use of navigation satellites. It would also require small thrusters with fuel to be used for minor orbital course corrections.

Function

The two objects start off in low circular or elliptical orbits, along the same trajectory but in opposite directions. Imagine the cylinder transcribing a path due north from the equator to the north pole and onwards around the planet, while the cigar transcribes the same path except in the opposite direction: heading southwards after it crosses the north pole. The two objects will thus intersect each time they complete a half-orbit.

As each vehicle circles the planet, it gathers electrical power from solar radiation using the attached photovoltaic panels. When the two orbits intersect, the electromagnets in the cigar and the cylinder are used so as to repel one another and increase the velocity of each projective, in opposite directions, by taking advantage of Newton’s third law of motion. Think of it being like a magnetically levitated train with a bit of track that gets pushed in the opposite direction, flies around the planet, and meets up with the train again. I warn you not to mock not the diagram of the craft! Graphic design is not my area of expertise. Obviously, it is not to scale.

The orbits

The diagram above demonstrates the path that one of the craft would take (see the second update below for more explanation). The dotted circle indicates where the two craft will meet for the first time, following the initial impulse. At that point, you could either project up to a higher elliptical orbit or circularize the orbit at that point. This process can be repeated over and over. Here is a version showing both craft, one in red and the other in brown. See also, this diagram of the Hohmann transfer orbit for the sake of comparison. The Hohmann transfer orbit is a method of raising a payload into a higher orbit using conventional thrusters.

The basic principle according to which these higher orbits are being achieved is akin to one being a bullet and the other being the gun. Because they have equal mass, the recoil would cause the same acceleration on the gun as it did on the bullet; they would start moving apart at equal velocity, in opposite directions. Because they can pass through one another, the ‘gun’ can be fired over and over. Because the power to do so comes from the sun, this can happen theoretically take place an infinite number of times, with a higher orbit generated after each.

Because each orbit is longer, the craft would intersect less and less frequently. This would be partially offset by the opportunity to collect more energy over the course of each orbit, for use during the boosting phase.

As such, orbit by orbit, the pair could climb farther and farther out of any gravity well in which it found itself: whether that of a planet, asteroid, or a star. Because the electromagnets could also be used in reverse, to slow the two projectiles equally, it could also ‘climb down’ into a lower orbit.

Applications

On planets like Earth, with thick atmospheres, such a system could only be used to lift payloads from low orbits achieved by other means to higher orbits. The benefit of that could be non-trivial, given that a low orbit takes place at about 700km and a geostationary orbit as used for communication and navigation satellites is at 35,790 km. Raising any mass to such an altitude requires formidable energy, despite the extent to which Earth’s gravity well becomes (exponentially) less powerful as the distance from the observer to the planet increases.

A system of such carriers could be used to shift materials from low to high orbit. The application here is especially exciting in airless or relatively airless environments. Ores mined from somewhere like the moon or an asteroid could be elevated in this way from a low starting point; with no atmosphere to get in the way, an orbit could be maintained at quite a low altitude above the surface.

Given a very long time period, such a device could even climb up through the gravity well that surrounds a star.

Problems

The first problem is one of accuracy. Making sure the two components would intersect with each orbit could be challenging. The magnets would have to be quite precisely aligned, and any small errors would need to be fixed so the craft would intersect properly. Because of sheer momentum, it would be an easier task with more massive craft. More massive vehicles would also take longer to rise in the gravity well through successive orbits, but would still require no fuel do so, beyond a minimal amount for correctional thrusters, which could be part of the payload.

Another problem could be that of time. I have done no calculations on how long it would take for such a device to climb from a low orbit to a high one. For raw ores, that might not matter very much. For satellite launches, it might matter rather more.

[Update: 11 August 2006] Many thanks to Mark Cummins for creating the orbital diagram I have added above. We are pretty confident that this one is correct. He describes it thus: “your first impulse sends you from the first circle into an elliptical orbit. When your two modules next meet, (half way round the ellipse), you can circularize your orbit and insert into the dotted circle, or you can keep “climbing”, an insert into a larger ellipse. Repeat ad infinitum until you are at the desired altitude, then circularize.”

Actually, I thought about this a little bit more, and I think I’m quite wrong about the shape of the orbit.
I think your system will quite happily execute a Hohmann transfer, as you suggest. I don’t know why I ever thought it wouldn’t.

Another potential problem that Mark and I came up with is that the gravitational pull of the moon might throw the orbits off, such that they will not intersect in the way portrayed above. I am not sure whether that’s true or not, but it would be a serious damper if it was.

Have you ever seen one of those big plastic funnels they use at science museums to demonstrate orbital physics? I bet you could make a small model of this thing that would zip around in one. It probably couldn’t generate enough power from the ambient light to overcome friction, but you could use batteries to boost it while still illustrating the point.

MonkeyClicker writes with mention of a proposal that could see an inflatable tower helping to carry people to the edge of space without the need for rocket propulsion. This would function in place of previous space elevator designs which featured a large cable and could be completed much faster, if proponents of the project are to be believed. “To stay upright and withstand winds, full-scale structures would require gyroscopes and active stabilization systems in each module. The team modeled a 15-kilometer tower made up of 100 modules, each one 150 meters tall and 230 meters in diameter, built from inflatable tubes 2 meters across. Quine estimates it would weigh about 800,000 tonnes when pressurized — around twice the weight of the world’s largest supertanker.”

Hugh Pickens writes “BBC reports that scientists are mapping the gravitational corridors created from the complex interplay of attractive forces between planets and moons that can be used to cut the cost of journeys in space. ‘Basically the idea is there are low energy pathways winding between planets and moons that would slash the amount of fuel needed to explore the solar system,’ says Professor Shane Ross from Virginia Tech. ‘These are free-fall pathways in space around and between gravitational bodies. Instead of falling down, like you do on Earth, you fall along these tubes.’ The pathways connect Lagrange points where gravitational forces balance out. Depicted by computer graphics, the pathways look like strands of spaghetti that wrap around planetary bodies and snake between them. ‘If you’re in a parking orbit round the Earth, and one of them intersects your trajectory, you just need enough fuel to change your velocity and now you’re on a new trajectory that is free,’ says Ross. ‘You could travel between the moons of Jupiter essentially for free. All you need is a little bit of fuel to do course corrections.’ The Genesis spacecraft used gravitational pathways that allowed the amount of fuel carried by the probe to be cut 10-fold, but the trade off is time. While it would take a few months to get around the Jovian moon system using gravitational currents (PDF), attempting to get a free ride from Earth to Mars on the currents might take thousands of years.”

Ion engines give a pretty feeble kick. Dawn’s produce 92 millinewtons of thrust, something like a fiftieth of the amount in a smallish firework rocket. The exhaust velocity, though, is enormous—more than ten times that of a chemical rocket—and this makes ion propulsion extremely efficient. Though an ion engine could never lift a spacecraft out of Earth’s gravity well, once that craft is in deep space the futuristic-looking blue glow of its exhaust can take it to parts that chemical engines find much harder to reach. Dawn started off with 450kg of propellant, and even at maximum throttle its engines use only a quarter of a kilo a day.